Magnetic recording media are widely used as a sound recording tape, video tape, computer tape, recording disc and the like. Ever-increasing demands have had to be met for increased magnetic recording density and shortened (higher frequency) recording wavelength. The recording system for these devices varies from an analogue system to a digital system. In response to the current demand for elevation of the magnetic recording density of the recording medium, a magnetic recording medium having a thin metal film, e.g., vapor deposition type, as the magnetic layer has been proposed. However, in view of the more facile productivity and practical reliability against corrosion or the like, a so-called coating type magnetic recording medium is nonetheless preferred. In a coating type magnetic recording medium, a dispersion of a ferromagnetic powder in a binder has been coated on the support. However, since a coated medium has a relatively low filling degree with respect to the magnetic substance, as compared with a medium having a thin metal film, the former is inferior to the latter with respect to the electromagnetic characteristic.
In any event, a widely used coating type magnetic recording medium comprises a dispersion of a ferromagnetic iron oxide, Co-modified ferromagnetic iron oxide powder, CrO.sub.2 powder or ferromagnetic alloy powder that has been coated on a non-magnetic support to form a magnetic layer thereon.
In order to improve the electromagnetic characteristic of such a coating type magnetic recording medium proposals have included, for example, improvement of the magnetic characteristic of the ferromagnetic powder added to the magnetic layer of the medium and smoothing of the surface of the medium. However, these proposed methods are not adequate to sufficiently elevate the magnetic recording density of the medium.
Recently, the recording wavelength for a coating type magnetic recording medium is being shortened concomitantly with elevation of the magnetic recording density of the medium. As a result, if the thickness of the coated magnetic layer is large, problems of self-demagnetization loss in recording with a lowered output arise and thickness loss in reproduction are serious.
Therefore, in response, reduction in the thickness of the magnetic layer has been attempted. If, however, the thickness of the magnetic layer is reduced to about 2 .mu.m or less, the surface of the magnetic layer would often be influenced by the non-magnetic support so that the electromagnetic characteristic and drop-out of the medium would worsen. However, the influence of the rough surface of the support would be avoided if a thick non-magnetic undercoating layer is provided on the surface of the support and then a magnetic layer is coated over the undercoating layer as an upper layer, as proposed in JP-A-57-198536 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). However, that method still has a problem in that the head abrasion resistance and the head durability worsen. The problem is considered attributable to the fact that a thermosetting resin is used as a binder in the lower non-magnetic layer. As a consequence, the lower non-magnetic layer is hardened so that the friction between the upper magnetic layer and head, as well as the contact of the upper magnetic layer with other parts, are effected under no buffer condition. The magnetic recording layer having such a lower non-magnetic layer has poor flexibility.
In order to avoid the problem, the use of a non-hardening resin as a binder in the lower non-magnetic layer is conceivable.
In accordance with the conventional method where the lower non-magnetic layer is coated and dried and then the magnetic layer is coated thereover as an upper layer, however, the lower non-magnetic layer would be swollen by the organic solvent in the coating solution for the upper magnetic layer to cause turbulence of the coating solution for the upper magnetic layer. As a result, the surface property of the upper magnetic layer would thereby be worsened and the electromagnetic characteristic of the medium would thereby be lowered. However, in order to reduce the thickness of the magnetic layer, reduction of the amount of the magnetic coating solution for the upper magnetic layer, or reduction of the concentration of the magnetic coating solution by adding a large amount of a solvent thereto, may be envisioned. In the former case of reducing the amount of the coating solution for the upper magnetic layer, however, drying of the coated layer would start before allowing sufficient leveling time to pass after the coating to cause another problem of coating defects of giving streaks or stamped patterns on the surface of the upper magnetic layer coated. As a result, the yield of the method would be extremely low.
On the other hand, in the latter case of using a magnetic coating solution having a low concentration, the coated film would be highly porous so that sufficient filling degree of a magnetic substance in the film could not be attained, and additionally, the strength of the coated film would be insufficient as the film is too porous. In short, both methods mentioned above have various unfavorable problems. Similarly, in the invention of JP-A-62-154225, it is insufficient in performance in respect to durability. This tendency becomes significant particularly when the thickness of the upper magnetic layers are less than 0.3 .mu.m.
As one means of overcoming the above problems, a method of simultaneous multi-coating system where a lower non-magnetic layer is provided and, while the lower non-magnetic layer is still wet, an upper magnetic layer containing a ferromagnetic powder is provided over the lower layer has been already proposed as described in U.S. Pat. No. 4,963,433 (corresponding to JP-A-63-191315) and U.S. Pat. No. 4,863,793 (corresponding to JP-A-63-187418). This technique yields a magnetic recording medium with high producibility, wherein the medium is free from coating defects and has elevated productivity, improved electromagnetic characteristics such as output and C/N ratio and improved running durability.
However, even using this method, when the thickness of the upper magnetic layer is less than 0.3 .mu.m, the strength of the magnetic layer is significantly decreased, leading to increases in simultaneous clogging and drop out. In order to solve such a problem of durability, it is proposed to add a large-sized filler to the lower non-magnetic layer, as described in JP-A-62-222427 and JP-A-2-257424. These proposals have however the drawback that the surface smoothness is insufficient.
In digital magnetic recording media which have come in practice recently, mutual interference takes place between adjacent reversals of magnetization in continuous high density reversals of magnetization, and a decrease in output peak value and a shift of a peak position occur to an isolated pulse waveform, which contributes to a detection error. The recording density is therefore prevented from being improved. In order to solve this problem and improve the recording density, it is necessary to reduce a half-value width of the isolated pulse waveform to decrease the interference of a symbol width. However, no coating type magnetic recording medium fully satisfying this demand has been obtained.
For this purpose, it is known that a decrease in the thickness of a magnetic layer, a decrease in Br/Hc (residual magnetic flux density/coercive force) and a reduction in surface roughness are effective. However, no example is known in which these are experimentally examined, for example, for a signal with a recording wavelength of 1 .mu.m or less in a coating type magnetic recording medium.
Furthermore, a decrease in the thickness of an upper magnetic layer deteriorates its adhesive property with a lower non-magnetic layer, so that the upper magnetic layer is liable to be separated from the lower non-magnetic layer. It becomes therefore difficult to ensure the running durability.
Accordingly, a coating type magnetic recording medium satisfying this demand has been desired, but no effective solution has been proposed yet.
For suitability with respect to digital recording, reference can be made to Katsuya Yokoyama, Guide to Magnetic Recording Technology (published by Sogo Denshi Shuppan, 1988).